We evaluated the effect of sample volume and citrate concentration on results of routine coagulation assays (prothrombin time [PT] and activated partial thromboplastin time [APTT]). The study was performed on samples obtained from healthy persons and patients receiving oral anticoagulant therapy. Standard evacuated tubes (3.2% and 3.8% sodium citrate) were filled to varying total sample volumes ranging from 3.0 to 5.0 mL, and results of routine coagulation tests were compared. Underfilling may significantly affect the APTT and PT, resulting in artifactual prolongation of results. This effect is most pronounced in samples drawn into 3.8% citrate. By using 3.8% citrate, there is a statistically significant difference in the results of PT assays in the samples less than 80% filled compared with those that are 100% filled. For APTT assays performed on samples drawn into 3.8% citrate, a statistical difference occurred at less than 90% filled. This effect was less pronounced when samples were drawn into 3.2% sodium citrate. We found no statistically significant difference in PT results from a 3.2% citrate tube between fill volumes of 60% and 100% and none for APTT results between fill volumes of 70% and 100%. This study further supports the recommendation to use 3.2% sodium citrate concentration, because 60% of the optimum filled volume for PT and 70% of the optimum filled volume for APTT are acceptable.